Each of co-pending U.S. patent application Ser. No. 658,303, filed Feb. 20, 1991, and U.S. Pat. No. 5, 038,567 discloses a geothermal power plant for use with a two-phase geothermal fluid source that produces a mixture of saturated steam and brine, the working fluid for the power plant being an organic fluid. In such a plant, a separator divides the geothermal fluid into a branch containing steam, and a branch containing brine. The steam is applied to a vaporizer containing an organic fluid that is vaporized by the steam which is cooled as a result, producing steam condensate. As explained in the '303 application, and in the '567 patent, the disclosures of which are hereby incorporated by reference, the cooled steam can be combined with the brine produced by its separator and applied to a preheater which serves to preheat the liquid organic fluid supplied to the vaporizer.
The organic vapor produced by the vaporizer is applied to a turbogenerator wherein the vapor expands producing electricity and heat depleted vaporized organic fluid. A condenser condenses the vaporized organic fluid producing condensate that is pumped back into the vaporizer through the preheater.
Typically, the minimum exit temperature of the brine from the preheater is limited by the minerals, chiefly silica, in the brine. This limits the amount of sensible heat that can be transferred from the brine to the liquid organic fluid in the preheater in order to prevent precipitation of the minerals in the brine. As a result, the exit temperature of the brine limits the power that the turbogenerator can produce. Furthermore, the amount of heat available in the steam usually exceeds the amount of heat needed to vaporize the organic fluid with the result that a significant amount of heat available in the geothermal fluid is not utilized. This is particularly true when organic fluids are used in sub-critical power cycles.
It is therefore an object of the present invention to provide a new and improved method of and apparatus for producing power using a two-phase geothermal fluid wherein the power output is increased without reducing either the exit temperature of the brine from the preheater, or the vaporizing temperature of the organic working fluid, even though the amount of heat utilized by the organic fluid is increased.